C file handling basics

1) General Pattern

• A FILE * is a handle the C library gives you so you can read from or write to a file.

• Typical pattern is: open -> use -> close.

  FILE *f = fopen("path.txt", "r");  // open
  /* read/write using f */
  fclose(f);                         // close
  

• If fopen fails (e.g., file missing or no permission), it returns NULL. Always check using something like if(!f) {error}

2) Common modes you’ll use

• "r" – open for reading (must already exist).
• "w" – open for writing (creates or truncates existing).
• "a" – append to end (creates if missing).
• Add "b" for binary (e.g., "rb", "wb"); on Linux/macOS it’s the same as text; on Windows it matters.

3) Reading lines

• fgets(buf, buf_size, file) reads one line (or up to buf_size-1 chars), including the newline if it fits.
• It returns NULL on EOF or error, which makes it perfect for a while loop.

4) Writing

• fprintf(file, "format", ...) works like printf, but the first argument is the file.
• Great for CSV or logs.

5) Always close

• fclose(file) flushes buffers and releases the handle. Do this for every file you open, can cause problems if not done correctly.

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Step‑by‑step example

Goal: Read numbers from numbers.txt (with a header line), compute average, and write results to average.txt in CSV.

#include <stdio.h>  // fopen, fgets, fprintf, fclose, printf
#include <stdlib.h> // atof

#define MAX_LINE    100   // max length of one input line
#define MAX_NUMBERS 100   // max numbers we’ll store

int main(void) {
    // 1) Open files
    FILE *input  = fopen("numbers.txt", "r");  // read text
    FILE *output = fopen("stats.txt",   "w");  // write text (truncate/create)

    // 2) Check they opened OK
    if (!input || !output) {
        printf("Error opening file.\n");
        return 1;
    }

    // 3) Prepare storage
    float numbers[MAX_NUMBERS];  // fixed-size array for numbers
    char  line[MAX_LINE];        // line buffer for fgets

    // 4) Skip the header line (we won’t use it)
    fgets(line, MAX_LINE, input);

    // 5) Read number lines until EOF or capacity reached
    int count = 0;
    while (fgets(line, MAX_LINE, input) && count < MAX_NUMBERS) {
        numbers[count] = atof(line);  // convert text line → float
        count++;
    }

    // 6) Compute sum, min, max across what we read
    float sum = 0.0f;
    for (int i = 0; i < count; i++) {
        sum += numbers[i];

    }

    // 7) Compute average
    float average = sum / count;

    // 8) Write results as CSV
    fprintf(output, "statistic,value\n");
    fprintf(output, "average,%.2f\n", average);

    // 9) Close files
    fclose(input);
    fclose(output);
    return 0;
}

Why each piece is there

• Two FILE*s: one for input, one for output—so you can read and write independently.
• fgets: safe, line‑oriented reading; it stops at newline or buffer limit and NUL‑terminates the string.
• atof: converts the line (text) into a float. This is a simple way to do this, there are nicer ways
• CSV output: using fprintf so spreadsheet apps can read it directly.
• fclose: finishes the job cleanly and ensures data is flushed to disk.

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Compile & run

gcc -Wall -Wextra -Wpedantic stats.c -o stats.out
./stats.out

Example files

numbers.txt (input)

value
12.5
3.0
7.25
-1.5
10

stats.txt (output)

statistic,value
average,6.65

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